Robert Downey Jr's next movie is going to be all about peddling goat testicles. Back up, read that last sentence again and start processing it.

Marvel's leading man will take a rare non-Iron Man acting role in Oscar-nominated Boyhood director Richard Linklater's adaptation of the controversial real-life story of medical pitchman John R. Brinkley.

Brinkley made a name for himself in the early days of radio for promoting supposed 'alternative' forms of medication to the American public - including injecting humans with goat testicles.